Alcohol consumption is a commonplace during festivities and also it is rather common that people do take to driving under drunken state and pose real danger to passer by and of the vehicles. As per laws of almost all the countries there is highest permissible limit of alcohol concentration in humans while driving. Not only this, a drunken subject is a cause of nuisance while on job and is endangered while working on a machine. All these situations warrant measurement of presence of alcohol in humans to meet the legal requirements. This social and legal objective is met by use of means which help in detection of alcohol.
Conventional technology utilized for alcohol (also referred to herein as ethanol or ethyl alcohol, CH3CH2OH) detection in traffic and traffic related situations relies on two different approaches: Screening for blood alcohol is used to determine whether an individual's blood alcohol content (BAC) is below or above a certain threshold value. In most cases, screening is done by means of breath analysis to establish the breath alcohol content (BAC). Evidential blood alcohol testing is required to establish legally-binding BAC values and, normally, is required after a positive breath alcohol test result.
There are a number of technologies that are used for alcohol detection in gas samples. The various means used in most commercial breath analyzers are: a) Fuel Cells b) Semiconductors c) Infrared Absorption d) Gas Chromatography and (e) Calorimetry. The use of semiconducting oxides as gas sensors has been common for several years. The electrical resistance of such sensors has been found to vary in a predictable manner when the sensor is operated in the presence of a particular gas, or concentration of gas, thus facilitating detection of particular gases or gas concentrations. Due to its high importances, there is a continuous need to prepare better and faster selective sensor for ethanol gas so that it can be used to test alcohol concentration in drunken subjects. This need has prompted the investigation of new materials/preparation by better technique which can function as gas detecting elements and particularly as alcohol sensors.
Stannic oxide has been found to be a particularly useful semiconductor oxide for gas detection when it is mixed with small amounts of a noble metal catalyst such as platinum, palladium and rhodium. This is disclosed in a U.S. Pat. No. 4,592,967. This invention discloses a gas sensor adapted to ethyl alcohol and the like and maintained at about 300.degree. to 450.degree. C. The draw back here is in the use of cumbersome process of making a solid paste and subsequent application on a substrate followed by high temperature firing at about 800 C for two hours. The method has been referred to as a kneading process.
Reference may also be made to a U.S. Pat. No. 5,944,661 which discloses electrochemical solid polymer electrolyte sensor for continuous ethanol measurement. The invention describes the continuous measurement of transdermal alcohol by measurement of electromechanical ethanol oxidation current. The transdermal alcohol sensor (TAS) essentially comprises a sensor assembly consisting of three-electrode system for measuring electrical signals. These electrodes are thermally processed in oven at 300-350° C. for a time in the range of 15-60 minutes. All the three electrode have to be bonded to the solid polymer electrolyte membrane at typical processing conditions of time in the range of 15-60 minutes, a temperature in the range of 250-350° C. and a pressure of 600 to 1200 psi. The major drawback of the TAS is that it has to form an airtight contact with the skin and also preferably needs a perspiring skin to actually effect the alcohol concentration measurement. Further drawback is that, to get a meaningful measurement, the sensor has to be used to record data over an extended period of time from few hours to a few days.
Reference may be made to yet another U.S. Pat. No. 5,907,407 and the PCT application number PCT/US99/17770. This invention describes alcohol sensing on the basis of intracavity laser spectroscopy (ILS) mainly for measuring alcohol in a vehicle. This is also a good means to detect consumption of alcohol by subjects driving vehicles under drunken state. However, the invention suffers from the drawback that a laser system is needed to be used. Yet another shortcoming is that the detection of alcohol in a vehicle by this or any other means may not warrant any legal action as the presence of an ethanol vapour can arise due to many other reasons not under the control of the driver.
Reference may be made to the work by Morrison, et al. (U.S. Pat. No. 5,082,789, 1992) which shows that bismuth molybdate (a term which is hereinafter used to describe an oxide where bismuth and molybdenum are cations of various atomic percentages and oxygen is the anion) can be used as a gas sensor with good sensitivity for certain gases and good reproducibility and stability, and in particular almost zero dependence of the sensor characteristics (the electrical resistivity) on the relative humidity. The patent particularly describes Bismuth molybdate gas sensors useful for the detection of alcohol in the breath, having both substantial sensitivity in the concentration range of interest (200 ppm) and having negligible response to the humidity from the breath. Bismuth molybdate sensors have been prepared in thin film of material or as a sintered powder. However, the invention suffers from certain drawbacks namely; the film material as grown by thermal evaporation was oxygen deficient and had to be sintered at 400° C. for 4 hours to improve the conductivity suitable for alcohol detection. The turn on time in the invention is also on a higher side i.e. 10 minute for a concentration of 200 ppm. The sensor operates at a temperature of 340° C. Further, to increase the sensitivity of the sensor, the invention proposes to dope the samples with noble metals like platinum, silver or palladium. The patent further discloses that the optimum sensitivity is obtained when the sample is a mixture of both, the  and  phases of bismuth molybdate. This situation is surely not desirable, as the control of the amount of presence of the two phases will be rather tricky and difficult. This may result in irreproducible desired sensitivity for the sample to act as ethanol or gas sensor. All these drawbacks combined together render the sensor, described in the patent, costly to manufacture and also rather problematic in use when a higher temperature is required for actual use.
Till now in most of the work to prepare bismuth molybdate one of the two techniques are followed, in the first method the oxides of bismuth and molybdenum are dry mixed and heated (calcined) at high temperatures and reacted to form compound (called the solid state or ceramic route). In the second method Bismuth molybdate is formed from aqueous solution by co-precipitation of bismuth and molybdenum oxide from bismuth nitrate and ammonium heptamolybdate, by adjusting the pH of the solution as disclosed in a U.S. Pat. No. 5,082,792. To prepare films either vacuum evaporation technique is used or the thick film procedure have been applied.
The present invention circumvents all the drawbacks as mentioned above and is capable of easy adaptation in any small scale manufacturing environment
To the best of our knowledge no patent exists on fabricating bismuth molybdate using metallorganic decomposition (MOD) route. The basic approach in MOD technique consists of simply dissolving the metal organic compound in a common solvent such as xylene and combing the solutions to yield the desired stoichiometry. Since the starting compounds are water insensitive, the solution retains proper stoichiometry. Once the deposition solution has been synthesized thin films can be prepared by known methods but other than evaporation techniques.